The present invention relates to a transmission system of a message (for example, process control data) between a plurality of processors or processing equipments (for example, computers or input/output devices) coupled to a common transmission line. Such a transmission system is disclosed in U.S. Pat. No. 4,366,479 issued to Mori et al, on Dec. 28, 1982. The present invention relates to an improvement of consistency check suitable to check redundant messages in the processors of such transmission system and select one among those messages.
In a distributed or dispersed processing method for processing a series of data by a plurality of processors connected to a common transmission line, if there are a plurality of processors which execute the same step, results of processing by those processors are sent to the transmission line as they are, and the receiving processor selects the messages relating to the same step from the received messages and they are determined by a majority decision in the receiving processor (JP-A-61-58050 laid open on Mar. 25, 1986).
In this method, messages having the same content code (code representing the content of the message) are collected for a certain period, the number of messages is counted, and they are determined by the majority decision.
In this method, the messages which belong to the same step of processing are identified only by the content codes. Thus, if a processor which executes the same step of processing is started by a different trigger (by interruption by an external unit) in a short period shorter than the message collection period, an output message by the different trigger cannot be identified.